In his SHOBOGENZO fascicle, SOKU SHIN ZE BUTSU (Our Mind is Buddha), Dogen says:
We realize that each and every aspect of existence is detached and
forms a unique independent existence, i.e., Buddha-nature. This is called
'Body and mind drop off.' This realization is dynamic, nothing like the static
existence of a Buddha statue. However, do not expect this truth to appear
easily without effort; without effort the truth remains hidden. 'Body and
mind drop off' represents universal truth, real existence in the present,
that neither reverts to the past, nor jumps ahead to the future.
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Joan Stambaugh replies:
This is strangely reminiscent of Heraclitus' "Nature loves to hide" [φύσις κρύπτεσθαι φιλεῖ]. Reality is "hidden," it keeps to itself. An
enormous effort is required to reach it; in fact, this "effort" somehow
belongs to reality and is integral to its manifestation.
Again, effort and sustained exertion, of which Dogen
speaks so often, are most obvious in the case of human beings, but there
is still no element of subjectivity involved here. For Dogen, the mountain
exerts itself, too. To speak in Heidegger's terminology, the mountain
presences; it is not just somehow there as an objectively present,
static object.For Dogen, mountains and rivers manifest and actualize the
Buddha-nature. Here at last is a thinker for whom "nature" is
central, not subordinated to "spirit" and inferior to it because
it lacks so-called "mind."